that WAS the case with previous GPU like the GTS 250 because the memory configuration is exactly 2x (like 512 MB and 1024 MB)
with the GTX 460, the memory configuration is not exactly 2x (768 MB 1024 MB) plus the bus speed on the GPU isn't the same (192-bit and 256-bit) and some of the L2 and ROP on the GPU is disabled for the 768 MB version. the GPUs are physically different. so you cannot SLI 1 GB with 768 MB.
this is confirmed although i'm trying to look for specific links.

now this USED to be ok with specific models like the GTS 250 that has different memory sizes but exact same GPU and bus. (i've SLI GTS 250 512 MB and GTS 250 1 GB before moving to this GTX 460).
but with the lastest 460, you CANNOT SLI models with different memory sizes.

My GTX 460 has been demoted to an overkill PhysX card. I caved and bought a GTX 570. I was going to originally go GTX 460 SLI, but no where local had the 768MB. Most games are right at the VRAM limit of those cards anyway nowadays, and I had a nice chunk of disposable income, so I went for it. Most expensive card I've ever bought. I also grabbed an e5700, originally for my media center, but I got it clocked to 4GHz, so it's now in my sig rig. I might do some benches with just the overclocked GTX 460 and e5700 @ 4GHz, if and when I feel up to it.

Question i have 2 460 gtx in sli i have the core up to 800 stable stock volts. one card was default 1900 mem other 1800 if i try to make them both 1900 i get crysis crashes at 1800 seem to be fine am i the only one unlucky to not be able to hit atleast 1900 mem